WO2007081548A2

WO2007081548A2 - Method of stretching a filled film to make it microporous and breathable

Info

Publication number: WO2007081548A2
Application number: PCT/US2006/049171
Authority: WO
Inventors: Michael Pickard; Gail Becke
Original assignee: Pliant Corporation
Priority date: 2006-01-06
Filing date: 2006-12-22
Publication date: 2007-07-19
Also published as: WO2007081548A3

Abstract

A method of making a microporous breathable film wherein a precursor film made from at least one polymeric material and a filler is heated and stretched between a single pair of textured stretching rollers. A positioning bar is disposed between the stretching rollers and is adjustable relative to the stretching rollers to control the strain rate and stretching of the film. The stretching rollers can be operated such that line speeds exceed about 2,000 feet per minute (fpm) (609.6 meters per minute), and the resulting stretched film is uniformly breathable with Moisture Vapor Transmission Rates (MVTR) that can exceed about 10,000 grams per square meter per day.

Description

METHOD OF STRETCHING A FILLED FILM TO MAKE IT MICROPOROUS AND

BREATHABLE

FIELD OF THE INVENTION

[0001] This invention relates to a method of making a film microporous and/or breathable. More particularly, the present invention relates to an improved method of stretching a filled film to make the film microporous and breathable, as well as to the breathable film made in accordance with the improved method.

BACKGROUND OF THE INVENTION

[0002] There exists in the art a variety of films which are capable of acting as a barrier layer, but which also allow air and water vapor to be transmitted through the film. Such films are known in the art as "breathable films."

[0003] One type of breathable film is a stretched filled microporous film. Such a film comprises a blend of polymeric material and incompatible organic fillers or inorganic particulate fillers which are extruded into a film or sheet. The film is then stretched or oriented, resulting in the creation of numerous interconnected pores throughout the film. These interconnected pores allow gas and water vapor to pass through the film while acting as a barrier to prevent the passage of fluids and solid matter through the film. The selection and amount of raw materials used to make the film, as well as the conditions under which stretching occurs, affects the density and the size of the pores, thus affecting the degree of breathability.

[0004J Machine-direction orientation (MDO) equipment for stretching filled films to induce breathability is known in the art. A typical MDO apparatus has a plurality of stretching rollers which progressively stretch and thin the filled film in the machine direction, i.e., in the direction of travel of the film. The rollers, which are heated to a desired stretching temperature, apply an amount of stress and progressively stretch the film to a stretching length where the film becomes microporous and breathable. While the apparatus generally is described as having five stretching rollers, the number of rollers may be greater or less depending on the level of stretch desired and the amount of stretching between each pair of rollers. Such MDO equipment is commercially available from, for example, Marshall and Williams Company, Providence, Rhode Island, and Black Clawson Converting Machinery, Fulton, New York. These commercial units typically cost about $1,000,000 for a 60-inch wide orienter.

[00051 Another limitation of MDO technology is the tendency to create thickened edges during the stretching process. The tensions across the web are such that stretching across the web is not uniform. In fact, it is not uncommon to see a 10 to 15 percent variation in film thickness across the width of the, web. Much of this variation is seen in the last few inches on either side of the width of the web. A factor in this phenomenon is the amount that the web "necks in" during orientation. As the level of "neck in" or width shrinkage in the web increases, the amount of variation in thickness across the web also increases. Due to this variation in thickness, it is common to remove the thick edges of the film and recycle them during the production of MDO film. The amount recycled can be up to 30 percent by weight of the total web. This is a source of tremendous inefficiency in the MDO process which heretofore has not been resolved.

[0006] There are also a number of patents that relate to MDO stretching or orienting a filled film to make it breathable. For example, U.S. Patent No. 5,017,323 to Balk discloses the use of two guide members in a preheated chamber in order to direct the path of the film and prevent it from prematurely stretching. To affect this result, Balk uses two different rollers, one of which has a high slip (or low coefficient of friction) surface, while the other has a low slip (or high coefficient of friction) or tacky surface. The speed of the second, low slip roller is limited to not more than the input speed of the film. The combination of the low speed and high coefficient of friction are effective for preventing the film from prematurely stretching.

[0007] Another patent relating to MDO stretching is U.S. Patent No. 6,179,939 to Jones, Jr., et al., which discloses a process in which the film is stretched in multiple discrete steps where each step is conducted with a strain rate of less than 100,000%/min. The process requires multiple pairs of stretching rollers in order to accomplish the multiple discrete stretching steps.

[0008] Although MDO equipment and processes are effective for producing breathable films, they suffer from several drawbacks. The biggest drawback is the high cost of the equipment. Because one of the primary uses for breathable film is in diapers, adult incontinence products, and feminine hygiene products, which are typically disposed of after a single use, cost is a predominant factor in the marketplace. Being able to cost-effectively produce breathable film for use in such disposable articles can mean the difference between producing films profitably or at a loss. Such cost-effectiveness is difficult to achieve with equipment that can cost about $1,000,000. Another drawback with MDO equipment is that it is typically large in size, making it difficult to place the equipment in-line with other processes such as film-making or diaper-making lines, making the end product less efficient to produce.

[0009] Another method of creating breathability in films is through the use of interdigitating rollers to incrementally stretch the film. Incrementally stretching a film using interdigitating rollers is described in U.S. Patent No. 4,285,100 to Schwarz. This patent discloses the process of stretching a film between two rollers each of which is provided with grooves that are parallel to the axis of the roller. The rollers are positioned so that the grooves of the rollers are intermeshed like gears. The rollers are used to stretch the film in both the machine and transverse directions. The patent further describes use of this process for both film and nonwoven substrates.

[0010] U.S. Patent No. 4,350,655 to Hoge describes the use of the interdigitating roller process with films having a formulation containing fillers. In the examples, Hoge indicates that these films become permeable to moisture vapor after being processed through interdigitating rollers.

[0011] Other, more recent patents relate to modifications to the use of interdigitating rollers for the production of breathable films. For example, U.S. Patent No. 6,013,151 to Wu describes a high speed method for creating microporous films that relies upon incremental stretching. A specific formulation using tri-block copolymer in addition to filler enables the high-speed extrusion and subsequent orientation of the film. In U.S. Patent No. 6,258,308 to Brady, the reference describes using heated interdigitating rollers at temperatures between 32.7° C (91° F) and 70.5° C (159° F) for production of breathable films. Brady also describes the ability to control breathability based on roll temperature. In U.S. Patent Nos. 6,264,864 and 6,706,228 to Mackay, each reference discloses a specific film formulation containing at least about 20 percent polypropylene in addition to inorganic filler. The film becomes breathable when stretched using interdigitating rollers held at temperatures between 35° C (95° F) and 60° C (140° F). In U.S. Patent No. 6,656,781 to Wu, the reference discloses interdigitating roller orientation of non-embossed films, where the non- embossed film results in higher breathability compared to an embossed film oriented at the same conditions. Further, in U.S. Patent No. 6,843,949, to, Brady, the reference discloses orienting films using interdigitating rollers that have been heated to between 35° C (95° F) and 70.5° C (159° F). In this Brady reference, the film may optionally be preheated prior to orientation. It may also optionally have a pattern of different thickness, resulting from embossment of the film prior to orientation. Finally, Brady describes the use of this process with composites of film and nonwoven.

[0012) Despite all of these advancements in the use of interdigitating rollers, one central problem remains. That is, that interdigitating rollers only stretch a small percentage of the film. Therefore, in order to create a certain level of overall breathability across the entire surface of the film, it is necessary to render the stretched areas much more breathable than the desired level in order to account for the large percentage of unstretched surface area. This requires stretching the film to a higher elongation in those areas. As a result, those areas of the film are weakened and are prone to failures. This is particularly important in the case of diaper backsheets, where superabsorbent polymer particles can puncture the weakened areas of the breathable film. An additional concern with weakened films of this nature is the possibility for a child or an infant to poke a fingernail through the film, thereby exposing the core material. In a small but significant number of cases, it has been reported that the child or infant then ingests the core material, which expands in the throat, ultimately causing death or serious injury.

[0013] A variant of incremental stretching that attempts to solve the problem of limited surface area stretching is U.S. Patent No. 6,811,643 to McAmish. This reference discloses combining at least one incremental stretching unit with one machine direction orientation unit. However, this continues to create a striated, and thereby weakened, film.

[0014] Prior art references that do not specify the process for creating breathability, beyond "stretching," also exist. These references leave the process open to various interpretations. For example, another variant of rendering films breathable is through stretching of a melt embossing film, as described in U.S. Patent No. 4,777,073 to Sheth. In this reference, melt embossing is performed prior to stretching so that the thinner areas of the embossed structure create higher breathability than do the thicker areas. Again, these thinner areas tend to be sources of weakness in the film. [0015] There is, therefore, a need in the art for an efficient, cost-effective method for producing breathable films that are not weakened by the stretching process. There is also a need in the market for a cost-effective breathable film that is free from weakened areas since the primary market for these films is in the disposable personal care and medical field.

SUMMARY OF THE INVENTION

[0016] One aspect of the presently described technology is directed to a method for making a stretched breathable film comprising the steps of (a) heating a precursor film comprising at least one thermoplastic polymer and a filler; and (b) continuously stretching the precursor film in a machine direction at a stretch ratio of at least 200% to form a stretched breathable film, or a stretched microporous breathable film. By continuously stretching the precursor film in the machine direction, the method of the presently described technology results in a uniformly breathable film across the entire surface of the film, free from weakened areas that can reduce the useful strength and puncture resistance of the film.

[0017] In another aspect, the present technology involves the precursor film being heated and continuously stretched in a machine direction between a single pair of textured rollers at a strain rate of greater than about 100,000%/min. The use of a single pair of stretching rollers instead of the plurality of stretching rollers typically used in machine direction orientation (MDO) equipment significantly reduces the cost of the equipment used in the method of the presently described technology. Moreover, the single pair of stretching rollers renders the equipment used in the method of the present technology smaller in size than typical MDO equipment and makes the equipment capable of running at line speeds in excess of 2,000 feet per minute (frjm) (609.6 meters per minute). The smaller size of the equipment allows the equipment to be installed directly onto other converting lines, such as diaper, adult incontinent or feminine hygiene converting lines, in order to render filled films breathable immediately prior to use in a finished article or in components of a finished article. The high line speed combined with the lower capital cost of the equipment and its ability to be used in-line with other converting lines results in a low cost, highly efficient production of breathable film.

[0018] A further aspect of the presently described technology is directed to breathable films made in accordance with the methods described herein. Such films can have a Moisture Vapor Transmission Rate (MVTR) in excess of about 5,000 grams per square meter per day, in excess of about 10,000 grams per square meter per day, or higher. In preferred embodiments, the films are uniformly breathable and substantially free from weakened areas that can reduce the useful strength and puncture resistance of the film. Further, in preferred embodiments, the films are produced without thick edges that need to be recycled. For example, films made according to the present technology preferably "neck in" (or shrink in the width dimension) by no more than about 20 percent during processing. As a result, the film thickness variation across the web is less than would be seen in a conventional MDO process. Generally, the film thickness variation across the web is less than about 15 percent overall and is free, or substantially free, from large variation in the area directly abutting the edges of the film. This can additionally result in the process(es) herein described being cost-effective for the production of breathable films.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a top view of a schematic diagram of a process line suitable for making a breathable film in accordance with the method of the present invention.

[0020] FIG. 2 is a perspective view of the process line of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Precursor films suitable for use in one of the presently described methods may be monolayer films or multilayer films having two or more layers. The films have at least one thermoplastic polymer and a filler.

[0022] Thermoplastic polymers useful for making the precursor films can be any thermoplastic polymer that is capable of being stretched in at least the machine direction. Typical thermoplastic polymers include, but are not limited to, polyolefins including homopolymers, copolymers, terpolymers and blends thereof. Examples of suitable thermoplastic polyolefins include polyethylenes, such as high density polyethylene (HDPE), low density polyethylene (LDPE), ultra low density polyethylene (ULDPE), and linear low density polyethylene (LLDPE), polypropylenes, ethylene-propylene copolymers, polymers made using a single-site catalyst, ethylene maleic anhydride copolymers (EMAs), ethylene vinyl acetate copolymers (EVAs), polymers made using Zeigler-Natta catalysts, styrene- containing block copolymers, blends and coextruded structures thereof, and derivatives thereof.

[0023] In addition to the thermoplastic polymer, the precursor film includes a filler to impart breathability to the film. The filler can be any material that is non-reactive with the thermoplastic polymer(s), is capable of being uniformly blended and dispersed into the thermoplastic polymer(s), and promotes a microporous structure within the film when the film is stretched. Both organic and inorganic materials are contemplated for use in the precursor film. Examples of useful fillers include alkali metal and alkaline earth metal carbonates, such as sodium carbonate (Na₂COs), calcium carbonate (CaCOs) and magnesium carbonate (MgCOj), non-swellable clays, silica (SiOa), magnesium sulfate, magnesium oxide, calcium oxide, alumina, mica, talc, titanium dioxide, zeolites, aluminum sulfate, aluminum hydroxide, barium sulfate, polymers, including high molecular weight high density polyethylene, polystyrene, nylon, blends thereof, derivatives thereof, and mixtures thereof. Use of polymer fillers creates "domains" within the thermoplastic polymer matrix. These domains are small areas, generally spherical, where only the polymer filler is present as compared to the remainder of the thermoplastic matrix where no polymer filler is present. As such, these domains act as particles and will be referred to as particles in this document.

[0024] Filler particles are desirably small in size in order to produce micropores in the film. Generally, the filler particles should have an average particle size in the range of about 0.1 microns to about 15 microns, preferably about 1 micron to about 5 microns, and more preferably about 1 micron to about 3 microns. In order to render the filler particles free- flowing and facilitate their dispersion in the polymeric material, the filler particles may be coated with a fatty acid or other suitable processing acid. Suitable fatty acids include, but are not limited to, stearic acid or a larger chain fatty acid.

[0025] In preferred embodiments, the filler content of the precursor film can be in the range of about 30% by weight to about 75% by weight of the precursor film. Although amounts of filler outside that range can be employed, an amount of filler that is less than about 30% by weight may not be sufficient to impart uniform breathability to the film. Amounts of filler greater than about 75% by weight may be difficult to blend with the polymer and may cause a loss in strength in the final breathable film. Particularly preferred amounts of filler are in the range of about 40% to about 60% by weight of the film, more preferably about 45% to about 55% by weight of the film. [0026] The precursor film may also contain other optional components to improve the film properties or processing of the film. Such optional components may include, for example, one or more anti-oxidants, which are typically added to reduce the tendency of the film to discolor over time, and one or more processing aids, which are added to facilitate extrusion of the precursor film. Amounts of anti-oxidants for use in the present technology are typically less than about 1% by weight of the film, and amounts of processing aids for use in the present technology are typically less than about 5% by weight of the film. Suitable anti-oxidants and processing aids are well known in this field. Other additives include, for example, whitening agents, such as titanium dioxide, which can be added to increase the opacity of the film. Whitening agents are typically added at levels of less than about 10% by weight of the film. Additional optional components include, but are not limited to antiblocking agents, such as diatomaceous earth, and slip agents, such as euracylamide, which can be added to enable film rolls to unwind properly and to facilitate processing through secondary process steps, such as diaper making. These are typically added at levels of less than about 5% by weight of the film. Additional additives may include, for example, scents, deodorizers, pigments other than white, noise reducing agents, among others, which are added at levels of less than about 10% by weight of the film.

[0027] The precursor film is typically prepared by mixing the thermoplastic polymer(s), the filler and any optional components together until thoroughly blended, heating the mixture, and then extruding the mixture to form the precursor film. A variety of film-forming processes known in the art may be used to form the precursor film. For example, the film may be manufactured by casting or extrusion using blown-film, coextrusion or free-film extrusion techniques. The precursor film may be wound onto a winder roll for subsequent stretching in accordance with the method of the presently described technology. Alternatively, the precursor film can be manufactured in-line with the film stretching apparatus described in more detail below.

[0028] Prior to stretching, the precursor film preferably has an initial basis weight of less than about 100 grams per square meter (gsm), and more preferably less than about 75 gsm. The precursor film may be a monolayer film, in which case the entire precursor film comprises the thermoplastic polymer(s) and the filler. The precursor film may also be a multilayer film, which typically comprises a core layer and one or more outer skin layers adjacent to the core layer. When the precursor film is a multilayer film, the core layer preferably comprises the thermoplastic polymer(s) and the filler, while the skin layer or layers may have the same or a different composition than that of the core. Typically, the skin layers are selected from a composition that can minimize the levels of volatiles building up on the extrusion die. Upon subsequent stretching, the core then becomes microporous and breathable, while the skin layers may or may not be breathable depending upon whether or not they contain a filler. Whether prescursor films of the present technology are monolayer or multilayer, they may be referred to herein as "filled" precursor films due to the presence of filler in at least the core layer. The thickness and composition of the skin layers are selected so that, when the precursor film is subsequently stretched, the resulting film is still breathable. To that end, the skin layers are relatively thin and preferably together comprise no more than about 30% of the total film thickness.

[0029] Turning now to FIGS. 1 and 2, there is shown a schematic diagram of an apparatus suitable for stretching a filled precursor film 10 in accordance with the method of the presently described technology. The precursor film 10 is unwound from a winder roll 12 via a positioning roll 14 and nip 15, and is guided to a stretching unit 20 by rotating guiding roll 16. The guiding roll 16 is preferably constructed from a low coefficient of friction material, such as steel or chrome-plated steel to facilitate transfer of the precursor film to the stretching unit 20. The precursor film is stretched between stretching rollers 22 and 24 in the stretching unit, as will be described in further detail below.

[0030] Before the film is stretched, however, it is heated in a heating zone 18 to a temperature below its melting point to allow the film to be stretched without breaking. The manner of heating the precursor film in the heating zone 18 is not particularly critical and it is contemplated that the heating can be accomplished in -a variety of ways. For example, the precursor film can be heated with heated blowers within the stretching unit 20. Alternatively, infrared heating units can be added to preheat the precursor film prior to its entry into the stretching unit 20 or within the stretching unit 20, or the guiding and stretching rollers themselves may be heated. Regardless of the manner in which the heating of the precursor film is accomplished, it is important that the precursor film not be heated so greatly that the film achieves a temperature greater than its melting point. In general, a preheated film temperature in the range of about 32.7° C (91° F) to about 204.4° C (400° F) is suitable for heating the precursor films produced of various formulations of the presently described technology. It is also preferable that the film be heated throughout its travel through the stretching unit 20. [0031] The precursor film 10 passes from the guiding roller 16 to the first stretching roller 22 within the stretching unit 20. Both stretching roller 22 and stretching roller 24 have a textured rubber surface to allow for excellent tracking of the film even at elevated film speeds of about 2,000 fpm (609.6 meters per minute), or greater than about 2,000 fpm (609.6 meters per minute). Examples of such textured rollers are described more fully in U.S. Patent No. 5,531,393 to Salzsauler, et al., which is herein incorporated by reference in its entirety. It is important to note that the textured stretching rollers 22 and 24 do not engage with each other in an "interdigitating" or "intermeshing" fashion. Thus, the stretching rollers 22 and 24 do not create weakened areas in the film as the film is stretched, which can occur with interdigitating or intermeshing rollers.

[0032] In the embodiment illustrated in FIG. 1, the first stretching roller 22 rotates about a first shaft 32 in a counterclockwise (relative to the drawing) direction, and the second stretching roller 24 likewise rotates about a second shaft 34 in a counterclockwise direction. Preferably, stretching rollers 22 and 24 have the same diameter. Disposed between the first and second stretching rollers 22, 24, is a positioning bar 26. The positioning bar 26 has a longitudinal axis and is adjustably mounted such that the longitudinal axis is parallel to each of the first shaft 32 and the second shaft 34 of the stretching rollers. The positioning bar aids in stretching the film and establishing the point at which stretching occurs.

[0033] Varying the position of the positioning bar 26 relative to the stretching rollers 22, 24 affects the properties of the resulting stretched film, in particular, the effective stretch ratio and resulting basis weight of the film. For example, it has been determined that when the positioning bar 26 is adjusted to a position that maximizes wrap on the first stretching roller 22 and minimizes the free film distance (also known as gap) before the second stretching roller 24, the effective stretch ratio is less than when the positioning bar 26 is adjusted to a position that reduces wrap on the first stretching roller 22 and increases the length of the free film path prior to the second stretching roller 24. Such a result is surprising because it is opposite of what one skilled in the art would expect based upon typical MDO equipment and oriented film process knowledge. Generally, in MDO and other oriented film processing, a longer free film path distance results in a lower strain rate and less stretching, whereas in the present technology, a longer free film path results in increased stretching. The effective stretch ratio of a film is defined as the basis weight of the precursor film divided by the basis weight of the stretched film. Strain rate is defined as the speed differential between the two stretching rollers (in percent) multiplied by the line speed (in feet per minute) and divided by the free film distance or gap (in inches, converted to feet).

[0034] The speeds of the first and second stretching rollers 22, 24 and the position of the positioning bar 26 are selected so as to cause the film to be stretched in the machine direction (i.e., in the direction of travel of the film). The stretching occurs at a strain rate exceeding about 100,000%/min. The speeds of the first and second stretching rollers 22 and 24 are selected such that the speed of the first stretching roller 22 is less than that of the second stretching roller 24. The speed of the first stretching roller can be in the range of about 1 fpm (.3 meters per minute) to about 5000 fpm (1524 meters per minute), preferably in the range of about 200 fpm (60.96 meters per minute) to about 1500 fpm (457.2 meters per minute). The speed of the second stretching roller 24 can be in the range of about 1 fpm (.3 meters per minute) to about 8000 fpm (2438.4 meters per minute), preferably in the range of about 1,500 fpm (457.2 meters per minute) to about 3,000 fpm (914.4 meters per minute). The speed of the first stretching roller 22 is controlled by a motor. A higher motor RPM results in faster line speed. Typically, the first stretching roller 22 and second stretching roller 24 are mechanically interlocked to each other through a gear. The difference in speed between the first and second stretching rollers can be controlled by the gear ratio used on the rollers. If, for example, the gear ratio is 4.48:1, and the first roller is rotating at 100 revolutions per minute, the second roller will be rotating at 448 revolutions per minute. Since both rollers are preferably the same diameter, the line speeds (in feet per minute or meters per minute) will be directly proportional to the speeds of revolution of the rollers. The gear ratio can be adjusted or controlled in a variety of ways known in the art. For example, each of the stretching rollers can be equipped with variable speed drives.

[0035] The position of the positioning bar 26 relative to the first and second stretching rollers can be adjusted in a variety of ways known in the art. For example, the positioning bar can be adjusted by attaching the end of the bar to a threaded rod, positioned perpendicularly to the positioning bar, and rotating the rod to move the bar forward and back. Preferably, the positioning bar is adjusted by attaching the end of the bar to a slide plate 28 and sliding the slide plate 28 forward or backward to move the positioning bar 26 closer or farther away from the stretching rollers 22, 24. The slide plate 28 can be moved by loosening a retaining nut, which is attached to a threaded screw in the slide plate, and manually moving the bar forward or backward, followed by re-tightening the retaining nut. Alternatively, the slide plate can be actuated by use of pneumatic cylinders, pivoted arms, or an electrical switch.

[0036] The relative spacing of the positioning bar 26 between the first and second stretching rollers, and also the relative spacing between the first and second stretching rollers 22, 24, respectively, are selected so as to minimize the "neck in" of the film as it is stretched between the stretching rollers. By controlling the relative spacing, "neck in" of the stretched film can be minimized so that it will be no more than about 20% of the width of the film. In general, "neck in" will be minimized if the relative spacing of the positioning bar from each of the stretching rollers is on the order of about 5 inches or less. While not wishing to be bound by any particular theory, it is believed that by minimizing the "neck in" of the stretching film, the resulting films have a fairly uniform thickness across the web of the film. Thus, films made in accordance with the present process preferably have a film thickness variation across the width of the web of less than about 15% overall and are free from a large thickness variation in the area directly abutting the edges of the film, resulting in substantially reduced film waste.

[0037] Stretching occurs between the positioning bar 26 and the second stretching roller 24 and causes the film to become uniformly opaque or "whitened." The whitening of the film occurs as a result of orientation of the filler material and the microporous voids formed within the film. As a result of the stretching between the positioning bar 26 and the second stretching roller 24, the film is stretched in the direction of travel at least about 200% of its original length, more preferably the film is stretched from about 200% to about 800%, and most preferably from about 250% to about 600% in order to impart breathability to the film. Depending upon the composition of the film, including the amount of filler, and the amount . of stretching, films made in accordance with the present technology can have a Moisture Vapor Transmission Rate of at least about 500 grams per square meter per day, at least about 1000 grams per square meter per day, at least about 5000 grams per square meter per day, or at least 10,000 grams per square meter per day.

[0038] After stretching, the film travels past a holding nip 40, which stabilizes the film, to a series of annealing rollers 42, 43, 44 and 45. Although annealing of the stretched film is optional, it is highly desirable to anneal the film after stretching in order to heat set the film so that it retains its shape. During the annealing step, the film is heated by the annealing rollers, which are preferably internally heated, to an annealing temperature in the range of about 65.5° C (150° F) to about 129.4° C (265° F). Desirably, the first annealing roller 42 is operated at a slightly higher speed than the speed of the second stretching roller 24, and each subsequent annealing roller 43, 44 and 45 is operated at a slower speed than the previous annealing roller such that annealing roller 45 is operated at a speed that is up to about 20% slower than the speed of the second stretching roller 24. Although four annealing rollers are illustrated, it will be appreciated that more or fewer annealing rollers could be employed.

[0039] After annealing, the stretched, annealed film can be cooled by a series of chilling rollers 48, 49 which are preferably internally water cooled. Cooling of the film allows the film to be wound onto a rewind roller without the risk of the wound film layers sticking to each other. Such sticking can damage the film during subsequent unwinding. The cooled film is then guided by guiding rollers 52 and 54 to a rewind roller 56, where the film is wound for subsequent use in a processing line. It will be appreciated that, in an alternative embodiment, the stretched, annealed film may be used directly in a converting line, such that the chilling and rewinding steps may be omitted.

[0040] The stretched films made in accordance with the presently described technology may be used in a variety of products, including diapers, adult incontinent products and feminine hygiene products, and other articles requiring a breathable barrier layer. Typically, the stretched films will be laminated to one or more support layers, such as nonwoven webs, or other webs, using conventional adhesive bonding or thermal bonding techniques known in the art.

[0041] It will be appreciated that the present technology can be operated either horizontally or vertically. It is also to be understood that the presently described process could be installed directly onto diaper, adult incontinent product or feminine hygiene product converting lines or other converting equipment in order to render filled films breathable immediately prior to use in a finished article or in components of a finished article. Because the present technology of stretching a filled film utilizes a single pair of stretching rollers to accomplish the stretching in a single step, the method is much more cost effective and efficient than prior art processes requiring multiple incremental stretching steps utilizing multiple pairs of stretching rollers.

[0042] Further, because the presently described technology continuously stretches the film over its entire surface, overall breathability of the film can be obtained at lower overall elongation levels compared to methods involving the use of interdigitating rollers or incremental stretching. Thus, it may be seen that the method of the present technology provides a cost-effective, efficient method for preparing a uniformly breathable film that is free from weakened areas.

[0043] In order to better understand the preferred embodiments and advantages of the present technology, reference may be had to the following examples. However, the examples should not be construed to limit the scope of the invention.

EXAMPLE I

[0044] A monolayer precursor film is prepared having the following formulation:

Component % by Weight

CaCO₃ 50.4%

Dow Chemical 2045 (LLDPE) 20.916%

Dow Chemical PL- 1280 (mPE) 12.6%

Chevron 9659 (HDPE) 15.0%

Standridge 26106 (process aid) 1.0%

CIBA B-900 (anti-oxidant) 0.084%

[0045] The precursor film has an initial basis weight of 66 grams per square meter (gsm). The initial basis weight is determined by cutting a 4 inch by 4 inch square of 10 plies of film, weighing the film plies in grams and dividing the film weight by film area in square meters. Using process equipment similar to that illustrated in FIG. 1, the precursor film is preheated at a temperature ranging from 128.8° C (264° F) to 135° C (275° F), and stretched between the first and second stretching rollers. The stretched films are evaluated for basis weight and breathability. Breathability is determined in accordance with ASTM D6701-01 using IOOK testing equipment available from Mocon/Modern Controls, Inc., Minneapolis, MN, and results are expressed as Moisture Vapor Transmission Rate (MVTR) in grams per square meter per day. Operating parameters and results are listed in Table 1 below. TABLE 1

[0046] All of the films show full whitening over the surface of the film, indicating stress- crazing and breathability without any weakened areas. From the results in Table 1 it may be seen that the films have an averaged basis weight of 20.6 gsm after stretching and have MVTR values ranging from 9,523 to 16,780, with an average of 11,923, thus indicating that films made in accordance with the present method are highly breathable.

EXAMPLE 2

J0047] A tri-layer precursor film having a central core layer and a skin layer on either side of the core layer is prepared. Each of the skin layers has a thickness that is 10% of the total thickness of the precursor film. The tri-layer film is prepared from the following formulation:

Component % hy Weight

CaCO₃ 51.74%

Dow Chemical 2045 (LLDPE) 21.47%

Dow Chemical PL-1280 (mPE) 12.94%

Chevron 9659 (HDPE) 9.8%

Chevron 1019 (LDPE skin) 2%

Ampacet 100458 (process aid) 1.96%

CIBA B-900 (anti oxidant) 0.09%

[0048] The tri-layer precursor film has an initial basis weight of 58 gsm. The film is heated and stretched using the same processing equipment that is used for Example 1. The stretched films are evaluated for basis weight and breathability in the same manner as Example 1. Operating parameters for the heating and stretching steps and the results from the stretching are listed in Table 2 below.

TABLE 2

10049] All of the films show full whitening over the surface of the film indicating breathability without weakened areas. From the results in Table 2 it may be seen that the films have an averaged basis weight of 16.7 gsm after stretching and have MVTR values ranging from 333 to 733, with an average of 486. Although these values are lower than those from Example I, the results indicate that even with non-filled (and therefore non- breathable) skin layers of 10%, the tri-layer films still have meaningful breathability.

EXAMPLE 3

[0050J A tri-layer precursor film is prepared in accordance with Example 2, except that the initial basis weight of the tri-layer film is 68 gsm. The film is heated and stretched using the same processing equipment that is used for Example 1. The stretched films are evaluated for basis weight and breathability in the same manner as Example 1. Operating parameters for the heating and stretching steps and the results from the stretching are listed in Table 3 below.

TABLE 3

[0051] AU of the films show full whitening over the entire surface of the film indicating breathability without weakened areas. From the results in Table 3, it may be seen that the films have an averaged basis weight of 15.7 gsrn after stretching and have MVTR values ranging from 334 to 531, with an average of 424. Even though the initial basis weight of the tri-layer film of Example 3 is greater than that of the tri-layer film of Example 2, the stretched basis weight of the Example 3 film is actually less than the stretched basis weight of the Example 2 film, indicating that the Example 3 film was stretched more than the Example 2 film. [0052] The difference in resulting basis weight and effective stretch ratio between Example 2 and Example 3 is attributable to adjusting the position of the positioning bar between the Example 2 and Example 3 runs. For the Example 3 run, the positioning bar is moved to a position closer to the first stretching roller, resulting in reduced wrap on the first stretching roller and added length to the free film path prior to the second stretching roller. Moving the positioning bar to this position results in increased stretching and an effective stretch ratio of about 4.3 for the Example 3 film. The difference in basis weight and effective stretch ratio for the Example 2 and Example 3 films is comparatively illustrated in the following Table 4:

TABLE 4

Initial Position of Stretched Effective Basis Preheat Motor Gear Positioning Basis Stretch Film Weight Temp. RPM Ratio Bar Weight Ratio

XP- 58 275° F 650 4.48:1 Minimize 17.2 3.37

8696 G (135° C) Gap roll l

XP- 58 275° F 800 4.48:1 Minimize 16.3 3.56

8696 G (135° C) Gap roll l

XP- 58 275° F 1200 4.48:1 Minimize 17.3 3.35

8696 G (135° C) Gap roll l

Average 3.43

XP- 68 275° F 375 4.48:1 Maximize 15.6 4.36 8696 G (135° C) Gap 68gsm

XP- 68 275° F 800 4.48:1 Maximize 15.5 4.39

8696 G (135° C) Gap roll 4

XP- 68 275° F 1420 4.48:1 Maximize 15.9 '4.28

8696G (135° C) Gap roll 4

Average 4.34 [0053] From the foregoing, it will be appreciated that although specific embodiments of the present technology have been described herein for purposes of illustration, various modifications may be made without deviating from the scope of the present invention.

Claims

1. A method of making a microporous breathable film comprising the steps of:

(a) heating a precursor film comprising a thermoplastic polymer and a filler; and

(b) continuously stretching the precursor film in a machine direction at a stretch ratio of at least 200% to form a stretched microporous breathable film.

2. The method of claim 1 , wherein the precursor film is heated at a temperature in the range of about 32.7° C (91° F) to about 204.4° C (400° F).

3. The method of claim 1, wherein the thermoplastic polymer comprises a polyolefin polymer.

4. The method of claim 1 , wherein the precursor film is stretched at a stretch ratio of about 250% to about 600%.

5. The method of claim 1 , wherein the precursor film is a monolayer film.

6. The method of claim 1, wherein the precursor film is a multilayer film comprising a core layer and at least one skin layer, wherein the core layer comprises a thermoplastic polymer and a filler.

7. The method of claim 1, wherein a single pair of stretching rollers, comprising a first stretching roller and a second stretching roller, is used to accomplish the stretching step, and the second stretching roller is operated at a faster speed than the first stretching roller.

8. The method of claim 7, wherein the first and second stretching rollers are textured rollers.

9. The method of claim 7, wherein a positioning bar is disposed between the first and second stretching rollers and is adjustable relative to the first and second stretching rollers.

10. The method of claim 1, further including the step of annealing the stretched film.

11. The method of claim 10, wherein the stretched film is cooled after the annealing step.

12. A method of making a microporous breathable film comprising the steps of:

(b) stretching the precursor film between a single pair of stretching rollers at a strain rate of greater than about 100,000%/min. to form a stretched breathable film.

13. The method of claim 12, wherein the precursor film is heated at a temperature in the range of about 32.7° C (91° F) to about 204.4° C (400° F).

14. The method of claim 12, wherein the single pair of stretching rollers comprises first and second textured rollers.

15. The method of claim 14, wherein the second textured roller is operated at a faster speed than that of the first textured roller.

16. The method of claim 15, wherein the speed of the second textured roller is from about 1500 fpm (457.2 meters per minute) to about 3000 fpm (914.4 meters per minute).

17. The method of claim 14, wherein a positioning bar is disposed between the first and second textured rollers and is adjustable relative to the first and second textured rollers.

18. The method of claim 12, wherein the precursor film is stretched at a stretch ratio of at least about 200%.

19. • The method of claim 18, wherein the precursor film is stretched at a stretch ratio of about 250% to about 600%.

20. The method of claim 12, wherein the thermoplastic polymer comprises a polyolefin polymer.

21. The method of claim 12, wherein the filler comprises calcium carbonate.

22. The method of claim 12, wherein the precursor film comprises from about 40% to about 60% by weight filler.

23. The method of claim 12, wherein the microporous breathable film has a thickness variation of less than about 15% across the width of the film.

24. The method of claim 12, wherein the microporous breathable film has a Moisture Vapor Transmission Rate of at least about 500 grams per square meter per day.

25. The method of claim 24, wherein the microporous breathable film has a Moisture Vapor Transmission Rate of at least about 1000 grams per square meter per day.

26. The method of claim 25, wherein the microporous breathable film has a Moisture Vapor Transmission Rate of at least about 5000 grams per square meter per day.

27. The method of claim 26, wherein the microporous breathable film has a Moisture Vapor Transmission Rate of at least about 10,000 grams per square meter per day.

28. The method of claim 12 wherein the filler is selected from the group consisting of alkali metal carbonates, alkaline earth metal carbonates, non-swellable clays, silica, magnesium sulfate, magnesium oxide, calcium oxide, alumina, mica, talc, titanium dioxide, zeolites, aluminum sulfate, aluminum hydroxide, barium sulfate, polymers, and mixtures thereof.